What are the hardware specifications required for installation

Good afternoon.
I apologize for the poor knowledge of the language - I use a translator.

Please tell us what server settings required for users ograzizatsii of 1100, almost all online and work through Zimbra Desktop or Web interface.(on-line min = 20%, max = 90%)
It is planned to use the system as a general in the otdnl organizer, briefcase.
Traffic-mail on the day of 1 GB

The total volume of the space is supposed 4.5 terabytes. So you plan to use RAID 10 from 4 to 3 TB of disk. = (4 * 3) / 2 = 6Tb.
15000rpm SAS drives, we can not buy because of cost.

The question remains open:
-What kind of CPU cycles to take and Number of
-How much RAM memory